NEW INSIGHTS INTO THE SOURCE AND EXTENT OF TURBIDITES ACROSS THE SOUTHERN CASCADIA SUBDUCTION MARGIN FROM OFFSHORE HOLOCENE RECORDS IN THE EEL RIVER FOREARC BASIN (Invited Presentation)

The spatial extent of deep-sea sediment gravity flow deposits correlated along the Cascadia subduction margin has led to an earthquake-triggered turbidite model used to infer earthquake magnitude. This assumes, however, that turbidity currents initiate within and are confined to canyon systems that transfer them down to the subduction front. Yet, the sources and sediment pathways of the abyssal turbidites are not well constrained and leave significant uncertainty as to whether turbidite records along the subduction zone reflect megathrust (≥M_w8) earthquakes that induce flows from shelf to trench, or rather only record failures proximal to the subduction front. New evidence from the Eel River forearc basin challenges the existing model and has implications for the magnitude and recurrence of megathrust earthquakes, particularly in southern Cascadia. The position of the low-gradient Eel River basin on the mid-slope appears to inhibit submarine canyon development and restrict sediment transfer from the shelf to deep sea throughout Holocene time. To investigate this, we collected a suite of sediment cores in the basin at water depths of ~900–1300 m along Chirp sub-bottom and Sparker multichannel seismic profiles that show thick, regionally extensive accumulations of late Pleistocene to Holocene sediment. We date 11 mid–late Holocene sand-rich beds, many of which are graded. Correlation of these turbidites with acoustic profiles suggest they are extensive. However, cores collected within and at the head of Klamath Canyon, located down-slope of Eel River basin, do not contain Holocene event deposits, suggesting a lack of connectivity of the basin to this lower-slope canyon. This indicates that the mid-slope Eel River basin serves as an effective sediment sink. These findings contradict models of upper-slope failures resulting in sediment transport down canyons to deep-water fan systems, instead forcing us to consider more proximal sources from the subduction front to abyssal turbidites. Ongoing investigations into the sources and sediment pathways of southern Cascadia turbidites include assessing additional cores obtained from adjacent canyon systems to further test synchronicity and extent of event deposits and re-evaluate the existing model for megathrust earthquakes and seismoturbidite generation.